1. Field of the Invention
The present invention relates to the field of radio communication apparatus such as mobile telephones, portable telephones, cordless telephones, and the like. More specifically, the present invention relates to a radio mobile unit used in a radio communication system in which the strength of a received signal is measured by the mobile unit.
2. Description of the Related Art
As the number of subscribers in cellular radio systems increase, it is desirable to incorporate digital signal transmission methods into the analog cellular system that are presently available in order to transmit speech signals at a more efficient transmission rate. A system of this type is called a Dual Mode Cellular System. In such a system, speech signals may be selectively transmitted between a base station and a mobile unit over speech radio links in either analog or digital mode. In the analog mode transmission, the speech signals are modulated by an analog modulation method, for example, by frequency modulation (FM).
In the digital mode transmission, the speech signals are encoded into digital signals and transmitted at a more efficient transmission rate. Still, in the digital mode, speech communication links may be established by the time division multiple access (TDMA) method between a mobile unit and a base unit. In the TDMA, signals to be transmitted to a particular mobile unit are transmitted in designated time slots of a radio channel. The technical standard for the dual mode cellular system is specified in "Dual-Mode Mobile Station-Base Station Compatibility Standard" published by the EIA (Electronic Industries Association) as IS-54.
In a dual mode mobile unit served by such a dual mode cellular system, a received signal strength indicator may be employed as has been employed in a conventional analog-only mobile unit. If a received signal strength measuring circuit device which is common to both of the digital and analog mode transmission is used, the way in which the device measures the signal strength in the digital mode is the same as that in the analog mode.
A deficiency of such a dual mode mobile unit is in that the signal strength measured by the unit varies depending on whether the unit operates in the analog mode or in the digital mode even if the unit is located in a position where it is a certain distance away from a base station. This is because the signals are received continuously in the analog mode while the signals are received intermittently in the digital mode owing to the TDMA method adopted in the digital mode. Therefore, even if the strength of received signals in the analog mode were substantially the same as that in the digital mode, an average value of the strength of signals measured in the digital mode is lower than that measured in the analog mode.
Another problem is that more accurate received signal strength information is required to be transmitted to a base station serving the mobile unit, in accordance with the technical standard. Specifically, the received signal strength information has to be included in the MAHO (Mobile Assisted Handoff) control signal which is defined in the technical standard. The MAHO control signal is transmitted to the base station in the digital mode operation. However, the received signal strength indicator employed in the conventional analog-only mobile unit, which consists of an integrated circuit device, is not capable of accurately measuring the received signal strength. This is because the integrated circuit device individually has measurement characteristics of its own when the device measures the received signal strength.